Recipients’ and Environmental Factors Affecting the Pregnancy Rates of a Large, Fresh In Vitro Fertilization-Embryo Transfer Program for Dairy Cows in a Commercial Herd in China

The main objective of this study was to determine the influence of the recipient dairy cows' breed, lactation number, estrus condition, the type, location and volume of the corpus luteum (CL) and the time of year that the embryo transfer (ET) was performed on the pregnancy rates of a large, fresh in vitro fertilization-embryo transfer program for dairy cows in a commercial herd in China. The recipients were from a herd of dairy cows in Ningxia, a province in northwest China, and we statistically analyzed the data of 495 cows from 2021 to 2023. Cumulus oocyte complexes (COCS) were isolated from follicular fluid obtained through ovum pick-up (OPU) and oocytes were incubated 20-22 h for in vitro maturation (IVM). Embryos were obtained after 10-12 h of in vitro fertilization (IVF) and six days of in vitro culture (IVC). Embryos at the morula or blastocyst stage were transferred to randomly chosen recipients (n = 495). The influence of recipients' breed (Holstein or other), recipients' lactation number (heifers or cows), estrus type (natural or synchronized), CL type (homogeneous, CL_hom or cavitary, CL_cav), CL side (left or right), volume of the CL and season of transfer (spring, autumn or winter) on pregnancy rates were determined. The pregnancy rates were analyzed by binomial logistic regression with IBM SPSS statistics software, version 26. Pregnancy rates after ET to Holstein cows and other breeds were 43.49% and 42.68%, respectively (<i>p</i> > 0.05). Regarding age, pregnancy rates were 45.56% for heifers and 30.77% for cows (<i>p</i> < 0.05). Pregnancy rates following ET during natural and synchronized estrus were 44.41% and 41.5%, respectively (<i>p</i> > 0.05). Pregnancy rates with a left- or right-side CL were 40.18% and 45.65%, respectively (<i>p</i> > 0.05). The pregnancy rates achieved with a CL_hom and CL_cav were 44.44% and 39.68%, respectively (<i>p</i> < 0.05). The rates obtained in spring, autumn and winter were 49.26%, 46.02% and 34.64%, respectively (<i>p</i> < 0.05). Moreover, it was found that pregnancy rates were higher in recipients with a CL volume measuring greater than 10 cm³ compared with those with a CL volume measuring less than 10 cm³ (<i>p</i> < 0.05). The comparisons showed that recipients' breed, estrus type or side of the CL had no effect, but the recipients' lactation number, ET season and the type and volume of the CL have significant effects on pregnancy rates during ET.